The need to purify gases treated in chemical processes, supplied to customers, or discharged to the atmosphere, from sulfur compounds, in particular hydrogen sulfide, is well known. Indeed, a number of processes are known, directed to the removal of hydrogen sulfide from gas.
In some of these processes, hydrogen sulfide is first concentrated by means of a liquid absorbent, whereafter the regenerated H.sub.2 S gas is converted into elemental sulfur, which is not harmful. In certain cases it is possible to omit the first step, i.e., concentrating the hydrogen sulfide, and to convert it direct into elemental sulfur. A requisite in many of these cases, then, is that the components in the gas which do not contain sulfur are not reacted. Such a process is called a selective oxidation process.
One of the best known methods of converting hydrogen sulfide to elemental sulfur is the so-called Claus process.
The Claus process is carried out in various ways, depending on the hydrogen sulfide content in the gas being treated. In one embodiment a portion of the hydrogen sulfide is combusted to form sulfur dioxide, which then reacts further with the remaining hydrogen sulfide to form elemental sulfur. A detailed description of the Claus process is to be found in R. N. Maddox, `Gas and Liquid Sweetening`; Campbell Petroleum Series (1977), p. 239-243, and also in H. G. Paskall, `Capability of the Modified Claus Process`, publ.: Western Research & Development, Calgary, Alberta, Canada (1979).
In the Claus process, however, the H.sub.2 S is not quantitatively converted into elemental sulfur, mainly as a result of the fact that the Claus reaction is not quite completed: EQU H.sub.2 S+SO.sub.2 2H.sub.2 O+3/n S.sub.n ( 1).
Accordingly, there are remaining amounts of H.sub.2 S and SO.sub.2. Now, the effluence of the H.sub.2 S containing residual gas is not permitted, so that this has hitherto been combusted, whereby the hydrogen sulfide and other sulfur compounds, and also the elemental sulfur present in the gaseous phase are oxidized to form sulfur dioxide. As environmental requirements are becoming stricter, this will no longer be permitted, in view of the too high emission of sulfur dioxide that would result. It is therefore necessary to process the residual gas of the Claus installation, the so-called tail gas, further in a so-called tail gas plant.
Tail-gas processes are known to those skilled in the art and described, inter alia, in NL-A-No. 7104155 and in B. G. Goar, `Tail Gas clean-up processes, a review`, paper at the 33rd Annual Gas Conditioning Conference, Norman, Okla., Mar. 7-9, 1983.
The best known and hitherto most effective process for the treatment of tail gas is the SCOT process. This process is described, for example, in Maddox, `Gas and liquid sweetening` (1977), publ. Campbell Petroleum Series, p. 280. In this process, the tail gas is passed with hydrogen over a cobalt molybdenum on Al.sub.2 O.sub.3 catalyst, whereby the SO.sub.2 present is catalytically reduced. The total quantity of H.sub.2 S is subsequently separated in the usual way by liquid absorbtion. This requires previous conversion of SO.sub.2 to H.sub.2 S, as the presence of SO.sub.2 is a very disturbing factor. One disadvantage of the SCOT process is, therefore, the need of using complicated equipment. A further disadvantage is the high energy consumption needed to regenerate the hydrogen sulfide from the absorbent.
Another possibility of converting hydrogen sulfide in tail gas to elemental sulfur is the so-called BSR Selectox process, which is described in U.S. Pat. No. 4,311,683.
In that process, the H.sub.2 S containing gas is mixed with oxygen and passed over a catalyst containing vanadium oxides and vanadium sulfides on a non-alkaline, porous, refractory oxidic carrier. The conversion is carried out at a temperature of between 121.degree. and 232.degree. C.
A major drawback of both the SCOT process and the BSR Selectox process is that in both cases, after the hydrogenation of the sulfur components present to H.sub.2 S, the tail gas must first be cooled to remove the major part of the water. In fact, water greatly interferes with the absorption and oxidation of H.sub.2 S. Owing to the high investments involved, the cost of the tail gas treated by these known processes is high.
Another process for the oxidation of H.sub.2 S to elemental sulfur is described in U.S. Pat. No. 3,393,050. According to that publication, the hydrogen sulfide containing gas is passed with an oxidixing gas over a suitable catalyst contained in the tubes of a so-called tubular reactor, with the tubes being externally cooled. The catalyst considered suitable is bauxite, aluminum oxide (gamma alumina) or aluminum silicate as described in U.S. Pat. No. 2,971,824. Apart from the disadvantages mentioned above, the effectiveness of this process, just as of the other known oxidation processes, is insufficient. Thus, for example, the data given in U.S. Pat. No. 4,311,683 show that the formation of SO.sub.2 cannot be avoided, in spite of the low temperature that is used. In view of the ratio of H.sub.2 S to SO.sub.2 found in the `product gas` it must be supposed that this formation of SO.sub.2 is at least partially connected with the at least partial establishment of the Claus equilibrium. Indeed, it is in particular the occurrence of the following side-reactions which adversely affect the effectiveness:
1. The continued oxidation of sulfur: ##EQU1##
2. The reverse (or rather reversing) Claus reversible reaction: ##EQU2## In it the sulfur once formed reacts with the water vapour that is also present to form hydrogen sulfide and sulfur dioxide.
3. The so-called sulfation of the catalyst, for example: EQU MeO+SO.sub.2 +1/2O.sub.2 .fwdarw.MeSO.sub.4 ( 4)
As a result of this reaction, metal oxides present in the catalyst are converted into sulfates, whereby the catalytic activity is reduced, sometimes even to a substantial extent.
4. The formation of SO.sub.3 (over certain metal oxides) according to EQU SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3 ( 5)
5. Pore condensation of sulfur formed in the catalyst bed, mainly owing to condensation in the catalyst pores (so-called capillary condensation) which may occur above the sulfur dew point.
The occurrence of the side reactions listed above is partially determined by conditions in practice.
Generally speaking, tail gas contains in addition to elemental sulfur a considerable concentration of water vapour, which concentration may range between 10 and 40% by volume. This water vapour greatly promotes the reversing Claus reaction. Substantial removal thereof has evident technological disadvantages, such as the need of an additional cooling/heating stage, an additional sulfur recovery stage, or a hydrogenation stage followed by a (water removing) quench stage. A process in which the selectivity is not affected by the water content of the gas would therefore be highly desirable.
Another important circumstance is that in selective oxidation processes, some excess of oxygen will generally be used, not only to prevent H.sub.2 S from `slipping through`, but also on the ground of considerations of control technology. It is this very excess of oxygen, however, which will give rise to the continued oxidation of the elemental sulfur formed, whereby the effectiveness of the process is adversely affected.